The xscreensaver program waits until the keyboard and mouse have been
idle for a period, and then runs a graphics demo chosen at random. It
turns off as soon as there is any mouse or keyboard activity. It can
also lock the screen (immediately, after a longer idle period, or on
demand.) xscreensaver consists of two parts: xscreensaver itself, the
``driver'' or ``daemon'', which detects idleness and does locking; and
the many graphics demos that are launched by xscreensaver.

Any X program that can draw on the root window can be used with
xscreensaver, regardless of how that program is written, or what
language it is written in, or what libraries it uses. The xscreensaver
daemon takes care of detecting when the user is idle, locking, and
checking passwords and all the other book-keeping; all the other
programs need to do is draw.

The benefit that xscreensaver has over the combination of the xlock and
xautolock programs is the ease with which new graphics hacks can be
installed.  You don't need to recompile (or even re-run) the
xscreensaver program to add a new display mode, you just change a config
file.
